1. Field of the Invention
The present invention relates to a polishing liquid supply apparatus usable for a chemical mechanical polishing (CMP) apparatus, which is usable in a semiconductor device production process for smoothing a surface of a semiconductor device.
2. Description of the Related Art
In response to the increasing degree of integration, it has become increasingly important to smooth a surface of a wafer of a semiconductor device during the production process thereof. The wafer surface can be smoothed by a CMP apparatus. When the CMP apparatus is used, the wafer surface can be smoothed by a chemical mechanical polishing method which utilizes an interaction of mechanical polishing by a polishing pad and a polishing agent contained in the polishing liquid or slurry and chemical etching by a solution of slurry.
Recently, a so-called dicing machine method and a trench method have been widely used, by which a patterned film is buried in a wafer formed of a metal, dielectric or other material which is different from the material of the film, and the film is treated with chemical mechanical polishing. As a result, a wafer having a desired pattern of film buried therein is formed.
In such chemical mechanical polishing, the chemical properties of a polishing liquid used need to be strictly controlled in such a manner that the rate of polishing the film material is appropriate. The pH of the polishing liquid, which is closely related to the polishing speed, is especially important.
Conventionally, there is an attempt to stabilize the amount of the polishing liquid supplied to a chemical mechanical polishing apparatus from a polishing liquid supply system.
For example, Japanese Laid-Open Publication No. 9-131660 describes a semiconductor device production apparatus 700 as shown in FIG. 7 including a chemical mechanical polishing apparatus. The semiconductor device production apparatus 700 includes a polishing liquid tank 701 for storing a polishing liquid 2 used for polishing a semiconductor wafer or the like, crude polishing liquid tanks 713a and 713b connected to the polishing liquid tank 701 respectively through pipes 711a and 711b and pumps 712a and 712b, a chemical mechanical polishing apparatus 716 connected to the polishing liquid tank 701 through a pipe 709 and a pump 710, and a waste liquid treating apparatus 717 connected to the polishing liquid tank 701 through a pipe 714 and a pump 715.
The polishing liquid tank 701 accommodates a liquid level sensor 704 for measuring the amount of the polishing liquid 2 and a stirring device 708 for appropriately stirring the polishing liquid 2. A control section 707 is connected to the liquid level sensor 704, the stirring device 708, and a pH sensor (not shown) accommodated in the chemical mechanical polishing apparatus 716. The pH sensor is provided on an adsorption plate (not shown) for adsorbing a wafer accommodated in the chemical mechanical polishing apparatus 716. The polishing liquid 2 in the polishing liquid tank 701 is supplied to the chemical mechanical polishing apparatus 716 by the pump 710 through the pipe 709.
Before the wafer is polished, the pH sensor measures the pH of the polishing liquid 2. The driving amount of the pump 710 is adjusted based on the pH measured, and thus the amount of the supplied polishing liquid 2 is controlled.
Japanese Laid-Open Publication No. 7-233933 describes a polishing liquid supply apparatus 800 shown in FIG. 8. The polishing liquid supply apparatus 800 includes a mixer 801 for mixing the polishing liquid 2 with an additive liquid, a polishing liquid tank 802 connected to the mixer 801, an additive liquid supply pipe 806 for supplying the additive liquid to the mixer 801 via a control valve 807, and two detection pipes 811 and 812 inserted into the polishing liquid tank 802 at a level difference of H. The detection pipes 811 and 812 respectively have air injection holes at bottom ends 813 and 814 thereof. The polishing liquid supply apparatus 800 further includes an air supply source 815 for supplying air to top ends of the detection pipes 811 and 812 at certain pressures respectively, a differential pressure detector 818 for detecting a difference in the air pressure between the detection pipes 811 and 812, and a control device 819 for controlling the opening angle of the control valve 807. When the difference in the air pressure detected by the differential pressure detector 818 is larger than a set value 820, the control device 819 increases the opening angle of the control valve 807; and when the difference in the air pressure detected by the differential pressure detector 818 is smaller than a set value 820, the control device 819 decreases the opening angle of the control valve 807.
The concentration of the polishing liquid 2 in the polishing liquid tank 802 is controlled by adjusting, by controlling the control valve 807, the amount of the additive liquid supplied to the mixer 801 based on the difference in the air pressure detected by the differential pressure detector 818.
The chemical mechanical polishing system 700 shown in FIG. 7 has the following problem. A portion for coupling the pipe 709 to the polishing liquid tank 701 and a portion for coupling the pipes 711a and 711b to the polishing liquid tank 701 do not have a structure for blocking the external air. Due to such a structure, a gas 703 contained in the polishing liquid tank 701, which is adjusted to have an appropriate concentration to be used for polishing, is exposed to the external air. Accordingly, the external air invades into the polishing liquid tank 701.
The polishing liquid supply apparatus 800 shown in FIG. 8 has the following problem. External air invades into the polishing liquid tank 801 through the injection air holes at the bottom ends 813 and 814 of the detection pipes 811 and 812.
The following problem occurs when these apparatuses are used to perform chemical mechanical polishing. When, for example, a polishing liquid containing cerium oxide (ceria) or the like as a polishing agent is used, the polishing liquid deteriorates the polishing characteristics thereof over time due to the change in pH thereof in the polishing liquid tank. Although it is possible to adjust the pH by adding and mixing more polishing liquid, it is difficult to improve the polishing characteristics once they are deteriorated.
In chemical mechanical polishing, a difference in the polishing rate of films of two or more different materials to be polished can be utilized. In such a case, when the pH of the polishing liquid is 7, which indicates the liquid is neutral, the pH of the liquid may sometimes exceed 7 over time. Then, the polishing rates of the films to be polished and the difference in the polishing rate are significantly changed. Thus, the obtained polishing characteristics are far from the desirable characteristics. For example, when a polishing liquid containing cerium oxide is used for polishing a film containing silicon oxide (SiO.sub.2) and silicon nitride (Si.sub.3 N.sub.4) and the pH or the polishing liquid exceeds 7, a polishing rate 32 of an Si.sub.3 N.sub.4 film increases as shown in FIG. 2 as well as a polishing rate 31 of an SiO.sub.2 film, resulting in the Si.sub.3 N.sub.4 film being unnecessarily polished.
In order to avoid such an undesirable effect, the capacity of the polishing liquid tank needs to be restricted so as to prevent the polishing liquid 2 from staying in the polishing liquid tank for an extended period of time. When the used amount of the polishing liquid 2 is excessively small, the polishing liquid 2 needs to be disposed of long before the life expectancy of the polishing liquid 2.